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I have a trilevel house that is about 50 years old with forced air heat and a fairly new furnace. Now that winter is here, we've discovered that there is no heat in the bedroom that is on the ground level (the level that is on a slab with no basement underneath). There is decent heat "upstream" in the laundry room and bathroom, but none coming in the bedroom. The register damper is not closed (actually, there is no damper on this register). Could it be clogged somehow under the slab? Is there a way to fix this without major excavation? The attached photo of the exterior shows the problem bedroom on the lower right, adjacent to the garage, with the window behind a tree.

You might try a drain snake to see if it's plugged or collapsed.
It maybe disconnected from the main trunk.
Otherwise you may need to contact a heating contractor to check it out and give you some options.

A drain snake is a good idea to check if it's collapsed or plugged somehow...thanks! Wish I had something with a camera so I could visualize it.

I know I have the heating duct on the floor--the cold air return is in the ceiling and functioning well (I checked it with a piece of paper to make sure it was the return...and it constantly sucks the door to this room open when the furnace kicks on). I don't think there's a damper in the trunk--it's under the slab. The damper between the furnace and the slab entry point is open.

Would it make sense to start with a duct cleaning service, or does this sound like more of a heating contractor problem?

Concerning the HVAC ductwork that runs under the slab--is that surrounded by concrete, or is it just sitting in the soil under the concrete slab? I'm wondering if tree roots, etc. ever get into this (There's a large Ash and large Oak tree within 15 feet of the room).

Turns out the heating duct is collapsed under the slab. :( Apparently it's not a terribly unusual thing in a house that's 50 years old. Options include tearing out the concrete floor across three rooms to replace the duct ($10,000+) or installing a electric heater in the room and going without A/C ($500). Hmmmm.